Contactless identification systems or so-called radio frequency identification (RFID) systems are gaining more and more importance particularly in the field of logistics, commerce and industrial production. Thus, contactless identification systems or RFID systems are implemented in these fields. Further applications of identification systems are related to the identification of food, persons and animals. In addition, the upcoming technology of Near Field Communication (NFC), also applies to the same technical field.
In particular, such systems are suitable for wireless data transmission in a fast manner without cable connections and comprise at least one reader/writer device referred to as base station and one or more communication devices referred to as transponder. Different types of transponders are known from the art, in particular depending on their energy supply. Some of said transponders do not have their own power supply and are therefore called passive transponders. Passive transponders take the electrical energy required for their own power supply from an electromagnetic field provided by the at least one base station. To this end, the transponder has to be moved into the electromagnetic field of the base station.
For data transmission a communication device is inductively coupled to said reader/writer device and comprises at least a first electronic circuit, usually a microchip connected to a large area coil that functions as an antenna. The first electronic circuit may be connected via an internal communication interface to a second electronic circuit, for example a Smart Card like a Subscriber Identity Module (SIM) module or something similar for an exchange of data between said first and second electronic circuits. If the communication device is moved into the electromagnetic field provided by the reader/writer device, a current is induced in the large area coil and the first electronic circuit is powered up.
In the technical field of Near Field Communication (NFC) the European Computer Manufacturers Association (ECMA) has standardized a communication interface referred to as Near Field Communication Wired Interface (NFC-WI), which specifies a digital wire interface between said first electronic circuit connected to the antenna and the second electronic circuit of said communication device. The first electronic circuit may be a front-end device that drives a first high frequency signal via Signal-Out data line and receives a second high frequency signal via a Signal-In data line of the communication interface. The second electronic circuit may be a transceiver unit that drives said second high frequency signal via said Signal-In data line and receives said first high frequency signal via said Signal-Out data line of the communication interface. NFC-WI specifies the timing and signal requirements of the Signal-In and Signal-Out data lines as well as the handshaking between the first and second electronic circuit via the communication interface.
The electrical energy required for the power supply of the first electronic circuit is taken from the electromagnetic field provided by the reader/writer device, whereas the second electronic circuit may be powered by the first electronic circuit via a accumulator unit or via additional wire lines of the communication interface. Particularly in the case of the second electronic circuit being realized as a SIM card having a 7816-interface comprising eight ports for wire lines, only two single inputs ports for wire lines are left for the implementation of said communication interface. However, for the implementation of said communication interface, said first and second data signal as well as a clock signal have to be transmitted. Additional wire lines for the power supply of the SIM card are needed if the power is supplied by the first electronic circuit.
A two-wire communication interface capable of supplying electrical power to an electrical circuit via at least one wire line is well known in the art referred to as 4/20 mA interface. In contactless RFID communication systems however, the energy taken from the electromagnetic field of an RFID communication system is not sufficient for driving such a 4/20 mA interface. Moreover, the current consumption of said 4/20 mA interface is too high to be driven by an accumulator unit and is therefore not suited for an application in contactless RFID communication systems.